1. Field of the Invention
The present invention relates in general to a hydraulic control system for a vehicular power transmitting mechanism, and more particular to a hydraulic control system including two solenoid-operated valves for controlling a belt-tensioning hydraulic pressure for applying a tension to a belt of a continuously variable transmission of a belt-and-pulley type, a transient coupling hydraulic pressure of a frictional coupling device for establishing a power transmitting path, and a line pressure used for operating various hydraulically operated devices.
2. Discussion of the Related Art
JP-2002-181175A discloses an example of a hydraulic control system for a vehicular power transmitting mechanism including a continuously variable transmission of a belt-and-pulley type the speed ratio of which is continuously variable, and a hydraulically operated frictional coupling device which is engaged, for running of a vehicle, to establish a power transmitting path a portion of which is defined by the belt-and-pulley type continuously variable transmission. This hydraulic control system includes a single solenoid-operated valve for controlling a belt-tensioning hydraulic pressure for tensioning the belt of the continuously variable transmission, a transient coupling hydraulic pressure applied to the frictional coupling device for engaging the frictional coupling device, and a line pressure used for operating various hydraulically operated devices.
Where the same solenoid-operated valve is used for controlling the belt-tensioning hydraulic pressure and the transient coupling hydraulic pressure, as described above, these hydraulic pressures must be controlled such that a torque capacity of the belt of the continuously variable transmission is larger than a torque capacity of the frictional coupling device placed in the engaged state, in order to prevent slipping of the belt. Accordingly, when the transient coupling hydraulic pressure of the frictional coupling device is relatively high, the belt-tensioning hydraulic pressure of the continuously variable transmission may be unnecessarily high, giving rise to a risk of an increase of a load acting on the belt, consequent deterioration of durability of the belt, and reduction of power transmitting efficiency of the continuously variable transmission. The line pressure which is also controlled by the same solenoid-operated valve must be controlled on the basis of a higher one of the belt-tensioning hydraulic pressure and a transmission-shifting hydraulic pressure. The graph of FIG. 6 indicates an example of changes of the belt-tensioning hydraulic pressure Pout, a transmission-shifting hydraulic pressure Pin and the line pressure PL in relation to change of a speed ratio γ of the continuously variable transmission. When the speed ratio γ is relatively low, that is, when the continuously variable transmission is placed in a relatively high-gear state, the transmission-shifting hydraulic pressure Pin is generally higher than the belt-tensioning hydraulic pressure Pout. Where the relationship between the line pressure PL and the belt-tensioning hydraulic pressure Pout is determined on the basis of the transmission-shifting hydraulic pressure Pin when the speed ratio γ is relatively low, the line pressure PL tends to be unnecessarily high when the speed ratio γ is in a medium-to-high range in which the belt-tensioning hydraulic pressure Pout is higher than the transmission-shifting hydraulic pressure Pin, that is, when the continuously variable transmission is placed in a medium- to low-gear state, as indicated by one-dot chain line in FIG. 6. This tendency results in reduction of energy efficiency of the hydraulic system, and deterioration of fuel economy of the vehicle. The hydraulic characteristics indicated in FIG. 6 vary depending upon the specific configuration of the belt-and-pulley type continuously variable transmission.